Elsevier

Journal of Membrane Science

Volume 524, 15 February 2017, Pages 428-435
Journal of Membrane Science

Ultra-thin MFI membranes for olefin/nitrogen separation

https://doi.org/10.1016/j.memsci.2016.11.077Get rights and content

Highlights

  • Olefin/nitrogen separation was studied with ultra-thin MFI membrane.

  • In separation of olefin/nitrogen, the membranes were hydrocarbons selective.

  • Very high selectivity was obtained for separation of propylene/nitrogen mixture.

  • Very high permeance was obtained for separation of propylene/nitrogen and ethylene/nitrogen mixtures.

  • The separation with high permeance was affected by concentration polarization.

Abstract

The recovery of light hydrocarbons such as propylene and ethylene from vent streams in polymer plants is desirable since it opens up for more efficient conversion of the monomers with high economic value. Consequently, polymer membrane vapour-gas separation systems have been used for this purpose for decades [1,2]. However, an alternative is zeolite membranes. In this work, ultra-thin MFI zeolite membranes (0.5 µm) were used to separate propylene or ethylene from binary 20/80 olefin/nitrogen mixtures at different temperatures. The membranes were olefin selective with high permeance at all investigated temperatures. At room temperature, the permeance of propylene was 22×10−7 mol m−2 s−1 Pa−1 and the separation factor was 43, which corresponds to a separation selectivity of around 80. For a mixture of 20 mol% ethylene in nitrogen, the maximum separation factor was 6 (corresponds to a separation selectivity of 8.4) at 277 K with an ethylene permeance of 57×10−7 mol m−2 s−1 Pa−1. The membrane selectivity was governed by more extensive adsorption of olefin, especially propylene, as compared to nitrogen. Comparing with ethylene, propylene has higher heat of adsorption, which probably caused the higher propylene/nitrogen selectivity compared to ethylene/nitrogen selectivity. The permeance and the selectivity for propylene were much higher than for commercial polymeric membranes. For ethylene, the permeance was much higher, and the selectivity was comparable to commercial polymeric membranes. Modelling showed that the pressure drop over the support limited the flux through the membranes especially at higher temperatures and in particular for the ethylene/nitrogen system with high flux. Further, modelling indicated that the result obtained at high temperatures, where the flux was high, was also affected by concentration polarization. However, for the propylene/nitrogen system at the optimum separation temperature, the pressure drop over the support and the concentration polarization were small. The results show that ultra-thin MFI zeolite membranes are promising candidates for light olefins/nitrogen separation in polymer plants.

Introduction

Vapour-gas separation systems emerged in petrochemical and refinery industries due to the high economic value of the recovered hydrocarbons. Recovery of light hydrocarbons from petrochemical process streams, for example the purge gas from a polypropylene or polyethylene plant resin degasser, is an interesting application. Typically, the purge stream from a polypropylene plant contains 20–30% C3+ hydrocarbons in nitrogen, and it would be of great value for the industry to recycle this part and use the monomers for polymer production instead of burning it [1], [3], [4].

Membrane separations are particularly appealing for gas purification due to their low energy consumption, good selectivity and low costs. From organic to inorganic, various materials of membranes have been evaluated for separation of organic vapour/gas mixtures. Silicone rubber is by far the most widely used membrane material for recovering hydrocarbons from vent streams, as summarized by Baker et al. [1]. Silicon rubber membranes are well-suited for separation of organic vapour/gas mixtures and the commercialization of this type of membrane continue to stimulate membrane invention and industrial demand. However, these membranes are quite thick (typically ca. 50–100 µm), which partially explains why these membranes show rather low permeances. Furthermore, the selectivity is only 8–12 for propylene/nitrogen and ethylene/nitrogen mixtures [1,5]. Consequently, large membrane area and many modules are needed, which increases the capital cost of the facility. Besides, the membranes show rather low stability, freshly made thin composite polymer membranes will often lose 50% of the permeance within two weeks in this application. Paul et al. have shown that the performance deterioration results from the reordering of the polymer chains in the membrane, thus reducing the number and size of free volume elements that contribute to gas permeation [6]. Therefore, a key challenge for polymer membranes is to increase the long term stability. Enhanced performance may be obtained by combinations of polymers with inorganic particles in mixed-matrix membranes, but difficulties, for example achieving good dispersion [7], are frequently encountered for this type of membranes.

Over the past decades, the development of inorganic membranes, especially zeolite membranes, has gained increased interest due to the potentially high stability, permeability and selectivity. MFI zeolite is one of the most explored zeolites for membrane applications. MFI zeolite has a well-defined system of pores with a size of ca. 0.55 nm in diameter. Furthermore, the Si/Al ratio of this zeolite can be varied considerably making it possible to tailor the properties to a large extent, for instance hydrophobicity and ion-exchange capacity, which could open up for using of the membranes in a broad range of liquid and gas mixture separation applications [8], [9]. However, to the best of our knowledge, no olefin/nitrogen separation by MFI membrane has been reported until now. Thus far, additionally, most reports on zeolite membranes described relatively thick zeolite films; consequently permeances are typically quite low, comparable to polymeric membranes. However, to be competitive, zeolite membranes must show much higher permeance than polymeric membranes [10].

Our group has developed a technique for preparing ultra-thin (ca. 500 nm) MFI zeolite membranes on open graded supports showing very high permeances e.g. a CO2 permeance of 93×10−7 mol m−2 s−1 Pa−1 for 50/50 CO2/H2 binary mixture with a separation factor of 16.2 [11]. In addition, very high fluxes have been reported for p-xylene [12] and also for ethanol and n-butanol [13], [14].

In the present work, we evaluated our ultra-thin MFI membranes for separation of propylene/nitrogen and ethylene/nitrogen mixtures (20 mol% olefins in nitrogen) for the first time. The separations were performed at different temperatures to identify the optimum separation conditions. The effect of concentration polarization and pressure drop over the support were estimated by modelling.

Section snippets

Membrane preparation and characterization

The membranes were prepared using a seeding method on masked supports as described in detail previously [15]. Porous graded α-alumina discs (Fraunhofer IKTS, Germany) with a diameter of 25 mm comprised of a 30 µm thick top layer with a pore size of 100 nm and a 3 mm thick base layer with a pore size of 3 µm were used as supports. The supports were masked with polymethylmethacrylate (PMMA, Mw=100 000 g mol−1, q=1.8, CM 205, Polykemi AB, Sweden) and a Fisher Tropsh wax (Sasolwax C105, Carbona AB,

Membrane characterization

Top view and cross-sectional SEM images of an as-synthesised MFI membrane are shown in Fig. 1. The top view shows that the film is continuous film and is comprised of well intergrown zeolite crystals with a maximum size of about 400 nm at the top surface of the membrane. No defects could be observed by SEM in the membranes. The cross-sectional image shows that the film appears to be rather even with a total thickness including invasion of around 420 nm. The XRD data in Fig. 1 shows that the film

Conclusions

Ultra-thin (0.5 µm) MFI zeolite membranes were used to separate propylene or ethylene from binary mixtures with nitrogen at different temperatures. The membrane showed a high performance for both separations. The highest propylene/nitrogen separation factor and separation selectivity were 43 and 80, respectively, coupled with a permeance of 22×10−7 mol m−2 s−1 Pa−1 at room temperature. The maximum separation factor and separation selectivity for ethylene/nitrogen gas mixture were 6 and 8.4,

Acknowledgements

The Swedish Energy Agency (Grant no. 38336-1) are gratefully acknowledged for financially supporting this work.

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